A disc brake device is conventionally known in the art, wherein the disc brake device has a brake disc to be rotated together with a vehicle wheel, a brake pad to be pushed against a sliding surface of the brake disc, and an actuator for pushing the brake pad toward the brake disc.
FIGS. 14A to 14C and 15 are cross sectional views showing a major portion of a disc brake device of a movable caliper type, in which brake fluid pressure is used. In the disc brake device of this movable caliper type, a movable caliper C is arranged such that a relative movement of the movable caliper C to a mounting member (not shown), which is fixed to a vehicle body, is allowed only in an axial direction (a right-left direction in the drawing) of a brake disc D.
When the brake fluid pressure is applied to a wheel cylinder W (in FIG. 15) provided in the movable caliper C, a piston Pis is moved toward an inside sliding surface of the brake disc D (in a leftward direction in the drawing), wherein the piston Pis is slidably and fluid-tightly arranged in the wheel cylinder W by a piston seal S.
Accordingly, an inner pad Pin, which is fixed to a left-hand end of the piston Pis, is pushed against the inside sliding surface of the brake disc D. At the same time, the movable caliper C is moved in the opposite direction to that of the piston Pis (in a rightward direction in the drawing) upon receiving a reaction force from the piston Pis. As a result, an outer pad Pout, which is integrally fixed to the movable caliper C, is pushed against an outside sliding surface of the brake disc D.
As above, when the brake fluid pressure is applied to the wheel cylinder W, the inner and outer pads Pin and Pout are respectively pushed to the inside and outside sliding surfaces of the brake disc D in the opposing direction. The brake disc D is thereby held by and between the inner and outer pads Pin and Pout in proportion to the applied brake fluid pressure, so that a braking force is applied to a vehicle wheel corresponding to the applied brake fluid pressure.
FIG. 14A shows positions of the inner pad Pin, the outer pad Pout (i.e. the caliper C), and the brake disc D in a normal condition, wherein the applied brake fluid pressure is released from the wheel cylinder.
As shown in FIG. 14A, in the normal condition of the brake disc apparatus, a space is formed not only between the inner pad Pin and the inside sliding surface of the brake disc D, but also between the outer pad Pout and the outside sliding surface of the brake disc D. In particular, the space is formed between the inner pad Pin and the inside sliding surface of the brake disc D due to the following reasons.
When the piston Pis is moved toward the inside sliding surface of the brake disc D (i.e. in the leftward direction in the drawing) by the brake fluid pressure applied to the wheel cylinder W, the piston seal S is elastically deformed and held at its elastically deformed shape, wherein the piston seal S is elastically deformed in the direction toward the inside sliding surface of the brake disc D. When the brake fluid pressure is released in this condition, the piston seal S is restored to its initial shape by a restoring force due to the elastic deformation.
The piston Pis is relatively largely moved by the restoring force of the piston seal S in the direction away from the inside sliding surface of the brake disc D (i.e. in the rightward direction in the drawing), so that the space is formed between the inner pad Pin and the inside sliding surface of the brake disc D.
In the above normal condition shown in FIG. 14A, a brake drag (unfavorable frictional sliding) of the brake disc D is not generally generated. The brake drag of the brake disc D refers to such a condition in which the vehicle wheel (i.e. the brake disc D) is rotated while a brake pad is partly in contact with a sliding surface of a brake disc. Accordingly, in the normal condition of the brake disc (FIG. 14A), unfavorable abrasion of the brake pad (the inner and outer pads Pin and Pout) as well as the sliding surface of the brake disc D can be suppressed. Furthermore, a fade for the brake pad does not occur, in the normal condition.
A temperature of the brake disc is increased by the frequent braking operation. A thermal inclination of brake disc may occur at the brake disc D, when the temperature of the brake disc D is extremely increased. The thermal inclination of brake disc refers to a phenomenon, in which the sliding surface of the brake disc D is inclined, for example, as disclosed in Japanese Patent Publication No. 2004-36657. The thermal inclination of brake disc is also simply referred to as the thermal inclination.
FIG. 14B shows positions of the inner pad Pin, the outer pad Pout (i.e. the caliper C), and the brake disc D in the above thermal inclination, wherein the applied brake fluid pressure is released from the wheel cylinder. As shown in FIG. 14B, the sliding surface of the brake disc D is inclined toward the outside of the vehicle body. As seen from FIG. 14B, the brake drag may easily occur at corners of the respective brake pads (the inner pad Pin, the outer pad Pout).
The temperature of the brake disc D is decreased when the vehicle runs without braking operation, wherein the brake disc D is cooled by travel wind. Then, the brake disc D is restored from the thermal inclination to the normal condition.
FIG. 14C (as well as FIG. 15, showing an enlarged major portion of FIG. 14C) shows positions of the inner pad Pin, the outer pad Pout (i.e. the caliper C), and the brake disc D in a restored condition, wherein the applied brake fluid pressure is released from the wheel cylinder. Namely, the brake disc D is restored from the thermal inclination shown in FIG. 14B to the restored condition, as a result that the vehicle has run without braking operation.
As shown in FIG. 14C (and FIG. 15), the space is formed between the outer pad Pout and the outside sliding surface of the brake disc D. This is because the sliding surface of the brake disc D (having been inclined outwardly) is gradually restored to its initial (normal) shape, as the temperature of the brake disc D is decreased, and the outside sliding surface of the brake disc D is separated from the outer pad Pout. In other wards, the brake drag between the outer pad Pout and the outside sliding surface of the brake disc D may not be easily generated.
On the other hand, the brake drag between the inner pad Pin and the inside sliding surface of the brake disc D may be easily generated in the restored condition. This is because the inside sliding surface of the brake disc D pushes the inner pad Pin in the rightward direction in the drawing (namely the inside sliding surface of the brake disc D is brought into contact with the inner pad Pin), when the sliding surface of the brake disc D (having been inclined outwardly) is gradually restored to its initial (normal) shape, as the temperature of the brake disc D is decreased. Furthermore, it is because there is no movement of the inner pad Pin (as well as the piston Pis) by the restoring force of the piston seal S in the rightward direction in the drawing. Accordingly, the unfavorable abrasion of the inner pad Pin as well as the inside sliding surface of the brake disc D is likely to occur. And the fade for the inner pad Pin is also likely to occur.
As above, the position of the brake pads (Pin and Pout) relative to the brake disc (D) in the restored condition (FIG. 14C) from the thermal inclination is different from the position of the normal condition (FIG. 14A), which may cause a problem of the brake drag of the brake pads against the brake disc.